1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to adjusting the track density by changing the position error signal (PES) algorithm when servo writing a disk drive from spiral tracks.
2. Description of the Prior Art
When manufacturing a disk drive, servo sectors 20–2N are written to a disk 4 which define a plurality of radially-spaced, concentric data tracks 6 as shown in the prior art disk format of FIG. 1. Each data track 6 is partitioned into a plurality of data sectors wherein the servo sectors 20–2N are considered “embedded” in the data sectors. Each, servo sector (e.g., servo sector 24) comprises a preamble 8 for synchronizing gain control and timing recovery, a sync mark 10 for synchronizing to a data field 12 comprising coarse head positioning information such as a track number, and servo bursts 14 which provide fine head positioning information. The coarse head position information is processed to position a head over a target track during a seek operation, and the servo bursts 14 are processed to maintain the head over a centerline of the target track while writing or reading data during a tracking operation.
The track density as determined from the width of each track 6 is typically optimized for each head/disk interface. For example, a prior art technique referred to as a SQUASH measurement may determine the combined width of the read/write elements and therefore the optimal track density for each disk surface. The servo bursts 14 are then written to each disk surface in response to the SQUASH measurement to attain the desired track density. The track density may also be adjusted over the disk radius to compensate for degradation in reproduction accuracy due to various factors. For example, the track density is typically decreased toward the outer diameter tracks where servo errors (track misregistration errors) are amplified due to the increase in linear velocity, windage, and disk flutter affects. The track density may also be decreased toward the inner diameter tracks to reduce inter-track interference caused by the YAW angle of the actuator arm, particularly in disk drives employing magnetoresistive (MR) heads wherein a gap exists between the read element and the write element.
In the past, external servo writers have been used to write the product servo sectors 20–2N to the disk surface during manufacturing. External servo writers employ extremely accurate head positioning mechanics, such as a laser interferometer, to ensure the product servo sectors 20–2N are written at the proper radial location from the outer diameter of the disk to the inner diameter of the disk, as well as to achieve the desired track density for each head/disk interface as well as over the radius of the disk. However, external servo writers are expensive and require a clean room environment so that a head positioning pin can be inserted into the head disk assembly (HDA) without contaminating the disk. Thus, external servo writers have become an expensive bottleneck in the disk drive manufacturing process.
The prior art has suggested various “self-servo” writing methods wherein the internal electronics of the disk drive are used to write the product servo sectors independent of an external servo writer. For example, U.S. Pat. No. 5,668,679 teaches a disk drive which performs a self-servo writing operation by writing a plurality of spiral tracks to the disk which are then processed to write the product servo sectors along a circular path. Each spiral track is written to the disk as a high frequency signal (with missing bits), wherein the position error signal (PES) for tracking is generated relative to time shifts in the detected location of the spiral tracks. However, the '679 patent does not disclose how to optimize the track density for each head/disk interface or over the radius of the disk.
There is, therefore, a need to optimize the track density in a disk drive when servo writing from spiral tracks.